1. Field of the Invention
This invention relates to the field of display devices commonly used to present messages on a dot matrix, and in particular relates to an improved method and apparatus for modifying such a display device to present a message that appears to move, or travel, across the display device.
2. Description of the Prior Art
Display devices are commonly known in which messages are presented statically as a matrix of dots. The dots are illuminated or not depending upon the state of row-drive signals applied simultaneously to all of the dots in each of Y rows of dots and upon the state of a column-drive signal selectively applied to a column of dots in the matrix. Typically, the row-drive information is applied to the Y row-drive lines serially in time, as a synchronized clocking signal sequences through each column of the display. Such a display is known from U.S. Pat. No. 3,742,483, to James A. Ogle, entitled "Video Display System Using Display Panel". The Ogle display panel comprises a large number of small dot-like gas-filled cells, each of which has a drive circuit which can be energized to pass current therethrough to cause the cell to grow.
Reference is also made to Bulletin No. 1166D, dated October, 1974, of the Burroughs Corporation, Detroit, Michigan. The Burroughs Bulletin describes a display panel having a 5.times.7 dot matrix character format with two columns of space between each character for their 16-character display. Each character is defined by a 6-bit code, and the characters are scanned in a scanning mode entering characters from left to right. The first character is entered in the left-most position, and subsequent characters are entered sequentially into the next available space to the right. After the last character is displayed, a reset pulse is generated to initiate a new scan.
Each character may be formed by allowing or not allowing the individual dots to be visible. Characters are separated by columns or rows of dots that are not visible. This produces a visible (light) letter or symbol on a non-visible (dark) background. The non-visible and visible dots could be reversed to produce dark letters on a light background. Similarly, the space separating characters can be a horizontal row of non-visible dots for a display that "travels" vertically, or a vertical column of non-visible dots for a display that "travels" horizontally.
The Ogle U.S. Patent and the Burroughs Bulletin relate to displays which statically display a message, even though that message is refreshed cycically by the scanning mode described in the Burroughs Bulletin. As a result, a message composed of many characters may be presented on a display that would, without modification and improvement, only be capable of displaying a limited portion of the message.
A static message has several disadvantages, among which are that it could be unnoticed because it is not eye-catching. Additionally, the length of a message to be displayed is limited by the width of the display panel. In display systems which are purposely designed to catch the eye of the viewer, such as stock market report displays, displays for changing information at airport terminals and the like, and electronic games, causing the message to appear to travel would be a substantial and desirable improvement.
It is known in the prior art to construct an apparatus for generating a traveling display. Some previous methods have used mechanical means to progressively move the characters displayed in a dot matrix across the screen. However, such a mechanical means is subject to wear and physical breakage. Other prior art methods employ electronic means for effecting the moving display, but these methods have been limited to displays on cathode ray tubes. One such arrangement is shown in U.S. Pat. No. 3,742,482 to Albrecht, et al, the patent entitled, "Method and Apparatus for Generating a Traveling Display".
The Albrecht et al system generates a traveling display on a cathode ray tube by intensifying an electronic beam as it scans horizontally across the face of the CRT. The characters to be displayed are stored in a memory means with space being provided in the memory for a number of buffer characters waiting to be displayed. At a predetermined time in each cycle of the display, a determination is made of the number of buffer characters then awaiting display. Each character is formed from a plurality of strokes. Depending upon the number of buffer characters awaiting display, the display is shifted left by a selected number of character strokes for each display cycle. By varying the number of strokes that the display is advanced for each cycle in this way, the rate at which the display is moved may be varied as a function of the rate at which data to be displayed is received. The shifting by a small number of strokes for each cycle gives the illusion that the information is flowing smoothly across the display. The concepts and apparatus of Albrecht et al do not, however, apply to a dot-matrix display panel.